The present invention generally relates to the use of ultrasonic pulses, or waves, to measure the properties of objects in the wave's path, and more particularly to the use of an ultrasonic wave source to determine both the thickness of pipes and tubes and the speed of sound within the same, both from the same set of recorded data and without recourse to assumed or predetermined values of properties unique to the pipe or tube being measured.
Various types of non-destructive measurement methods have been employed in the art for determining an object's structural properties. One method, utilizing ultrasonic waves, is particularly appealing due to its accuracy, relative low cost, and safety. It is commonly used to determine object thickness, as well as to detect flaws and discontinuities. Accordingly, its use in inspecting objects with hollow internal cavities not readily amenable to visual perception or the placement of a conventional measuring device is especially valuable. Exploitation of the wavelike nature of acoustic signals, when combined with knowledge of the constitutive properties of the material making up the object, leads to reasonably accurate measurement of the object. Ultrasonic testing is well-suited to the measurement of material thickness, including layered or nonhomogenous materials with acoustically disparate properties. By taking advantage of different reflective properties of the constituent materials and the speed of propagation of the sound wave therethrough, the thickness of the material can be readily calculated.
The operation of a conventional ultrasonic measurement system in the time domain is relatively straightforward. Typically, a pulse generator produces an electrical signal with certain characteristics. This signal is sent to one or more transducers, where the signal is converted to an ultrasonic wave, which is then transmitted in the direction of a target object to be measured. Typically, both the target object as well as the transducers are in an acoustically-coupled medium, such as water, to enhance the transmission of the waves relative to a more rarified medium, such as air. Echoes reflected from the object return to the transducers, which convert the echo into a corresponding electrical signal, which is then routed to a receiver, where the signals can be counted or digitized, analyzed and stored. Analysis of the echo shows that the thickness of the object can be equated to a product of the speed of sound and the propagation time of the ultrasound wave within the object. Once any two of these three values are known, the third can be easily calculated. Nevertheless, the accuracy of some calculations based on time of flight data is subject to limitations. For example, in the conventional analysis discussed above, the speed of sound in the object being measured is assumed. Furthermore, even if the assumed speed was accurate for one temperature, it might not be for another. In addition, in the conventional analysis, the frequency-dependence of the speed of sound is not considered; the spectral content can become significant if the object being measured exhibits dispersive behavior, for example, where the speed of sound is frequency-dependent. These assumptions and simplifications, based on a constant, predetermined value, may introduce errors into subsequent thickness calculations. Moreover, for objects with multiple walls (such as a tube), there are limitations on placement of the ultrasonic transducers, especially where the inside diameter of the tube is small. Existing methods, while appropriate for geometrically simple structures, such as flat plates, are incapable of measuring individual wall thickness of multi-walled objects.
Current methods to measure the wall thickness of tubes and tubular-shaped objects either depend on an assumed sound velocity in the tube, or use a calibration procedure. Both of these approaches have disadvantages. In the former case, inaccuracies can result, either from improper characterization of the constitutive properties of the material in the object, or from inhomogeneities in the material itself, such as due to the presence of cladding, alloying or composite structures. Insofar as the equations used to calculate thickness depend on the speed of sound in the object being measured, any inaccuracies in that assumed quantity will produce errors in thickness calculations. In the latter case, calibration can be complicated and unreliable. In addition, it often must be conducted off-line, thereby taking away from precious measurement time.
It is well-known in the art to compensate measurements for variations in the temperature of the acoustic couplant medium (typically water). However, the temperature of the water can be significantly different than the temperature of the object in the water, especially when the object is passing through the water as part of a manufacturing step, such as the extrusion of tubes and related objects. Thus, assigning a temperature value (such as the measured temperature of the water in which the object is placed) to an object being measured may not accurately reflect the true temperature within the object.
The time-domain method provides a single scalar value of the speed of sound. If the sound wave passes through a dispersive medium, its determination will depend on the frequency characteristics of the transducers used in the measurement. However, an inherent part of the time-domain method is that such frequency-dependent values are not made manifest. Thus, in certain circumstances, additional measurement accuracy can be realized by using frequency-domain analysis, which can determine the phase velocity and group velocity at different frequencies. In the present context, the term “speed”, although in the strictest sense a scalar quantity, is used generally to represent both scalar and vector quantities in either time-domain or frequency-domain analyses. Contrarily, the more specific terms “phase velocity” and “group velocity” are both functions of frequency, and their use is restricted to frequency-domain analyses. The frequency-dependent quantities, while usually not as big of a contributor to a thorough and accurate determination of object thickness as the speed of sound in the object, can nonetheless provide additional insight into secondary levels of measurement error, especially when conducted on highly dispersive materials.
Accordingly, what is needed is a method for measuring object thickness and sound velocity through the object simultaneously, such that neither assumed properties nor the use of complicated calibration procedures is required. What is further needed is a method that maintains high accuracy in measuring the thickness of a highly dispersive object.